The exemplary embodiment relates generally to electrophotographic reproduction machines and, more particularly, concerns a method and apparatus for controlling the velocity of copy substrates during substrate registration in an electrophotographic reproduction machine, such as a printer or copier.
In high-speed reproduction machines, such as electrophotographic copiers and printers, a photoconductive member (or photoreceptor) is charged to a uniform potential and then a light image of an original document is exposed onto a photoconductive surface, either directly or via a digital image driven laser. Exposing the charged photoreceptor to a light image discharges the photoconductive surface thereof in areas corresponding to non-image areas in the original document while maintaining the charge on the image areas to create an electrostatic latent image of the original document on the photoconductive surface of the photoreceptor. A developer material is then brought into contact with the surface of the photoconductive member to transform the latent image into a visible reproduction. The developer material includes toner particles with an electrical polarity opposite that of the photoconductive member, causing them to be naturally drawn to it. A blank copy substrate such as a sheet of paper is brought into contact with the photoconductive member and the toner materials are transferred to it by electrostatic charging of the substrate. The substrate is subsequently heated for permanent bonding of the reproduced image, thus producing a hard copy reproduction of the original document or image. Thereafter, the photoconductive member is cleaned and reused for subsequent copy production.
Various sizes of copy substrates are typically stored in trays that are mounted at the side of the machine. In order to duplicate a document, a copy substrate with the appropriate dimensions is transported from the appropriate tray into the paper path just ahead of the photoreceptor. The substrate is then brought into contact with the toner image on the surface of the photoconductive member prior to transfer. However, a registration mechanism typically intercepts the substrate in advance of the photoconductive member and either stops it or slows it down in order to synchronize the substrate with the image on the photoconductive member. The registration mechanism also effects proper process direction (or longitudinal) alignment of the copy substrate prior to delivery to the photoconductive member by correcting skew in the substrate. The registration mechanism also effects proper cross-process direction (or lateral) alignment of the copy substrate prior to delivery to the photoconductive member by correcting lateral offset in the substrate.
One way to perform substrate registration is with a translational electronic registration (or TELER) system. A TELER system typically includes optical sensors, coaxial independently driven drive rollers (or nips), a carriage with a linear drive on which the independently driven paper drive rollers are mounted, and a microprocessor controller. In operation, a substrate is driven into the nips and moved through the paper path for placement and fusing of an image onto the substrate. The speed of both nips can be controlled to effect skew alignment and longitudinal registration. The nips are mounted on the carriage movable transversely with respect to the feed path. An optical sensor system controls positioning of the carriage to achieve the desired top edge or a lateral positioning of the substrate. Independent control of the nips and carriage translation provides simultaneous alignment in longitudinal and lateral directions.
Generally, in TELER-based systems and as shown in FIG. 1, the copy substrate travels to the registration nips at a given process velocity vp for the time period tproc−tdecel. It is decelerated at time tdecel to a given transfer velocity vt to complete registration and synchronize with the photoconductive member, which is also traveling at the transfer velocity Vt. This known velocity profile allows the image-to-substrate transfer to occur without smearing. However, due to recent developments in high-speed electrophotographic reproduction machines, the paper path must be able to transport and register ever smaller substrates, such as those less than letter (8½ by 11 inch) size, at increasingly faster speeds. In order to handle the smaller substrates and faster speeds, it has been found to be necessary to move the registration nips closer to the image transfer area. However, using the standard velocity profile as shown in FIG. 1, whereby the substrate is decelerated directly from the process velocity to the transfer velocity over the reduced nip-to-transfer distance, results in undesirable cross-process direction latitude. This is due primarily to the lack of time available to complete registration of the substrate.
Accordingly, there is a need for a method and apparatus for controlling the velocity of copy substrates during registration and allowing sufficient time for completing registration.